Susan's mother Grace comes to stay, and, because Ben has neglected to mend the toilet seat she falls off and is incapacitated, having to stay at the Harpers'. To avoid her Ben goes to visit Nick in his squalid flat but when he gets home he accidentally climbs into bed with his mother-in-law, who is in the master bed. Eventually everybody goes to Nick's to avoid Grace whilst Susan discovers a condom in Michael's wallet.
—don @ minifie-1